The invention relates generally to imaging systems and, more particularly, to techniques for processing image signals in an imaging system.
Since the advent of low light imaging, concerns have centered on the ability to enhance or stretch low light level details within a single image frame while preserving higher light level information within the same image. In the past, fixed, non-linear routines were used that amplified the low level signal detail to a greater extent than the higher level signal detail. These fixed algorithms provide some degree of image enhancement, primarily in the low light portions of the image, but are not capable of dynamically adjusting to changing scene content (e.g., changes in the relative amount of light and dark content). In addition, these prior systems often use a xe2x80x9cfirst orderxe2x80x9d correction function (e.g., a gamma function) that seriously compresses the high light level detail, thus reducing contrast in the image. Other prior art systems have used a feedback approach to enhance signal quality by feeding back a signal from the signal processing circuitry to the image sensor circuitry for use in adjusting the sensor settings based on image content. While such feedback systems can improve signal quality significantly, the delays caused by the feedback loop often result in slow, choppy video operation characterized by frequent blankouts and the like.
Therefore, there is a need for a method and apparatus for enhancing low intensity detail in an electronic image signal without adversely effecting the high intensity detail of the image. Preferably, the method and apparatus will be capable of enhancing both the low and the high intensity detail. In addition, there is a need for a method and apparatus for processing an image signal that is capable of automatically adjusting to changes in scene content without the use of signal feedback techniques.
The present invention relates to an image processing technique that is capable of enhancing both high and low level detail in an image signal simultaneously. In addition, the image processing technique automatically adjusts the type of processing applied to the image signal based on the content (i.e., the dynamic range) of the input image. The processing technique of the present invention is particularly suited for use in low-light-level imaging systems where a typical scene includes a large amount of low intensity detail. Occasionally, a low-light-level scene will have one or more high intensity point sources of light (e.g., headlights on an automobile) that significantly increase the overall dynamic range of the scene, but which add relatively little mid-intensity detail to the scene. The principles of the present invention allow such low light level scenes to be processed in a manner that enhances both the low light level detail and the high light level detail (when present) at the expense of the middle level portions of the scene. In addition, the type of processing that is applied to the input image will automatically adjust as the dynamic range of the scene varies (e.g., as point sources of light appear and disappear from the scene). This automatic adjustment is achieved without the use of feedback between the processing circuitry and the sensing circuitry, thus avoiding the problems often associated with such feedback techniques.
A sensor subsystem is provided that senses an external image and converts the image to an electrical representation having a peak value that is indicative of the dynamic range of the image. The electrical image signal is then applied to an electronic image processing subsystem having a unique multiple order, non-linear transfer function. The transfer function of the electronic image processing subsystem includes a first portion for modifying low level components of the image signal using gains in a first gain range, a second portion for modifying medium level components of the image signal using gains in a second gain range, and a third portion for modifying high level components of the image signal using gains in a third gain range. Both the first and third gain range are higher than the second gain range so that high and low level components in the image signal are expanded while medium level components are compressed. The multiple order, non-linear transfer function of the electronic image processing subsystem can be implemented using either analog or digital circuitry.
Because the sensor subsystem outputs an image signal having a peak value that is indicative of the dynamic range of the image, the type of processing that is applied using the transfer function of the electronic image processing subsystem will automatically adjust to the dynamic range of the image. That is, low dynamic range signals will be processed solely by the first portion of the transfer function, medium dynamic range signals will be processed by both the first and second portions of the transfer function, and high dynamic range signals will be processed using all three portions of the transfer function. Thus, the third portion of the transfer function will only be used when the difference between the lowest level detail and the highest level detail in the scene exceeds a predetermined value (e.g., a low light level scene with one or more point sources of light).
In a preferred embodiment of the invention, the first and second portions of the transfer function utilize a non-linear transfer characteristic that provides elevated gain to lower level input signal components while providing a more compressed gain as input levels rise. The third portion of the transfer function utilizes a linear transfer characteristic that provides a relatively large linear gain to the high level input signal components. This transfer function can be generated, for example, using a log amplifier, a linear amplifier, and a selection unit. The log amplifier and the linear amplifier each receive the electronic image signal from the sensor subsystem and amplify the signal according to respective transfer profiles. The selection unit then selects the larger of the output signals from the two amplifiers to generate the output signal of the electronic image processing subsystem. By properly selecting the gain characteristics of the two amplifiers, an overall transfer function can be generated that has the desired shape.
The output of the electronic image processing subsystem is applied to an automatic gain control (AGC) unit that modifies the signal so that a peak amplitude of the signal assumes a predetermined value. Preferably, the AGC unit will be a linear device that applies equal gain to all input signal levels. The output image signal of the AGC unit is delivered to a storage/display unit for storage and/or display.